1. Field of the Invention
The invention relates to a device for the frequency transposition of optical signals, namely a device enabling a wavelength transposition to be carried out on the signals.
The present invention finds applications in optical telecommunications systems, especially packet transmission networks, namely in optically frequency-transposed wavelength multiplexed packet transmission networks. They may be synchronous frames or asynchronous packets conveyed at different carrier lengths.
The performance characteristics of the optical fiber as a transmission medium mean that it is possible to envisage the conveyance of information over increasingly greater distances. The development of optical components for the transmission of modulated signals, reception as well as the development of optical amplifiers have led to the designing of high bit rate networks that seek to share the very wide transmission band offered by the optical fiber with several channels, each channel using the band necessary for the electronic systems.
This principle has served as the basis for proposals for optical wavelength multiplexed local networks, the designing of optical access networks combining optical multiplexing and synchronous temporal electronic multiplexing and optical packet networks in asynchronous transfer mode applications.
In many studies, it is proposed to share the transmission passband of the optical fiber among different channels that are represented by a specific optical frequency per channel.
The use of this technique, known as optical frequency division multiplexing (OFDM) or wavelength division multiplexing (WDM) is warranted by the discovery of optical functions enabling the processing of optical channels: filtering, extraction, insertion of channels, demultiplexing and wavelength conversion. The last-named technique of wavelength conversion enables the signal to be transferred from one carrier wavelength to another.
The usefulness of wavelength conversion depends on the type of network and the rules of management of the wavelengths used. However it can easily be seen that it offers great flexibility in the management of the channels, especially in the context of switched networks (synchronous or asynchronous) when the phenomenon of contention has to be avoided. This phenomenon corresponds to the blocking that arises when two signals of the same wavelength are conveyed by distinct physical media of the networks and, after passage into a switching node, have to be transmitted on the same physical medium.
Wavelength conversion also facilitates the reutilization of the wavelengths when two networks have to be connected. In short, it makes it possible to consider the field of the optical frequencies as a perfectly flexible resource as regards its management.
2. Description of the Prior Art
The techniques available today can be used most often to obtain efficient wavelength conversion over a wide range of wavelength (namely over a wide possible range for (.lambda.0, .lambda.1 . . . .lambda.n), transparent to the bit rate conveyed on the wavelengths and independently of the state of polarization of the incident signals. However, to date, there are no simple techniques enabling the "pseudo-transposition" of .lambda.0 to .lambda.0 for example without causing deterioration in the quality of the optical signal.
Now, this problem is encountered when it is desired to achieve an optical frequency transposition and when, among the input signals to be transposed, there are signals which are at the transposition wavelength. This may occur in the case of the transmission of wavelength multiplexed packets.
What has to be done therefore is to convert the signals at wavelengths of .lambda.0, .lambda.1 . . . .lambda.n into a wavelength of .lambda.0. Devices to achieve this function have already been designed. However, the use of one of these devices without any particular precaution entails the risk of considerably modifying the quality of the signals that arrive at the wavelength .lambda.0.
One of the known techniques of wavelength transposition that is simplest to implement makes use of the compression of the gain of the semiconductor amplifiers.
A schematic drawing of an experimental device is shown in FIG. 1. Continuous power P.sub.c at the wavelength .lambda.0 (probe) is injected at the same time as the intensity modulated P.sub.m signal .lambda.i into a semiconductor optical amplifier. At output P.sub.T of the amplifier and filtering means 20 and 21, the signal of the probe is modulated by the variations of the gain which are related to the variations of intensity of the incident signal (.lambda.i). Should .lambda.0=.lambda.i, the continuous signal injected (probe) appears as a spurious signal. The rate of extinction of the optical signal is therefore deteriorated.
One approach could consist in modulating the pump signal so as not to inject it when .lambda.0=.lambda.i. This approach however is difficult to implement because of problems of synchronization. Indeed, it is necessary to know the time at which the probe must be extinguished to let through the packet or the frame at the wavelength .lambda.0.